Direct-acting cholinergic agonists comprise choline esters such as acetylcholine, methacholine and bethanechol and alkaloids such as pilocarpine and cevimeline. They have diverse therapeutic applications. During eye surgery, acetylcholine is used to constrict the pupils. However, its application is limited by its non-selectivity and temporary effects. Methacholine stimulates muscarinic receptors in the airways. Its inhalation is used to diagnose bronchial airway hyperreactivity. The degree of airway narrowing helps diagnose asthma and its severity. Another muscarinic stimulator, bethanechol, improves gastrointestinal and urinary tract contractions, relieving constipation and the inability to urinate. Eye drops containing pilocarpine, a muscarinic and ganglionic stimulator, are used to lower intraocular pressure and treat glaucoma. Cevimeline is a selective M3 agonist. It is used to help patients with dry mouth or eyes by increasing salivation and lacrimal secretion.